1. Field of the Invention
The invention relates to a method for controlling the output of a load connected to ac line voltage. The invention further relates to an arrangement for controlling the output of the load pursuant to the inventive method.
2. Description of the Prior Art
Devices for controlling the output of loads which are connected to ac voltage, generally the line voltage, are known in a variety of forms. Such devices usually comprise a phase control circuit by which the blocking period or the firing angle can be adjusted in a desired manner at the a.c. line voltage supplied to the consumer, so that the supplied power can be regulated.
Phase control circuits of this kind are known, for example, from German reference DE 33 03 126 C2, which is directed to a device for switch-on peak limiting in a motor control unit for the driving motor of a vacuum cleaner. The motor control unit is provided with a phase control circuit. German reference DE 43 27 070 C1 describes a device for regulating the power consumption of a vacuum cleaner in which the ac voltage supplied to the electric motor driving the vacuum cleaner is regulated via a phase control circuit such that the supply voltage value corresponds to the effective value of the motor voltage. The phase control circuits used for this purpose generally contain a triac which is connected to line voltage in series with the load, in this case an electric motor, and supplies the electric motor with a discontinuous (sine) voltage depending upon the power desired.
Based on a circuit built from discrete components for the phase control, although the range of conceivable possibilities for realizing the circuit extends from optionally large-scale integration to pure microprocessor control, the phase control in the trigger circuit for the triac comprises a resistor which is generally adjustable as a potentiometer or trimmer and a charging capacitor for firing the triac depending on the adjusted resistance, so that practically any desired intermediate outputs up to the full angle can be called up as desired by appropriate shifting of the firing angle.
However, a recent problem in this regard in phase control circuits of this kind is that with increasingly higher possible maximum power output of the electrical load, a limit is predetermined with respect to the harmonics generated overall by the load, including its control circuit. This limit can not be overcome by simple means. For the sake of clarity, the output of a vacuum cleaner motor will be taken as an example hereinafter, although it is understood that the invention is applicable to any electrical load.
In general, harmonics always occur in the absence of proportionality between the current and voltage, wherein, in the output control of an electric universal motor, harmonics are chiefly formed at first as odd-numbered harmonics which are derived in a first approximation from the quadratic dependency between current and voltage. Additional harmonics are formed through the phase control itself. These harmonics are especially pronounced when the firing angle is approximately 90.degree., that is, when the permeability to current of the series triac is effected to some extent in the middle of the respective half-wave as a result of corresponding firing.
The above-mentioned limit of permissible harmonic content is determined by governmental regulations and is represented in Europe by the EMV (EMC or electromagnetic compatibility) standard, as it is called, and to this extent also represents a power limit for the permissible motor output which, expressed in numerical values, is roughly 1200 to 1400 W based in general on conventional electric motors with the known phase controls.
The problems resulting from this limit can be circumvented or combatted only at a high cost. The measurements are conventionally carried out at a phase angle of 90.degree. and with reference to 16 successive full waves, during which time the harmonic content may not exceed a predetermined value.
Therefore, it is also known to avoid this problem by way of hardware, namely by way of the electric motor, by using a motor with two field windings. One of the windings is available for power delivery up to a maximum 1400 W, for example, which can be operated via a phase control with a harmonic content just within the permissible range. At an increased demand for power, the other field winding is switched to by suitable switching means (microswitch) while the phase control is completely switched off, so that full power, e.g., 1800 W, can be achieved with the second field winding in continuous full-wave operation, that is, without phase control. Naturally, the harmonic content attributable to the phase control is dispensed with in its entirety.
Such measures which are elaborate with respect to construction require considerable additional material at correspondingly high costs, not the least of which is made up by assembly costs, and necessitate omission of the phase control precisely when it would be especially desirable under certain circumstances in the very high power output range for sensitive adjustment.